Plutonium enriched uranium fuel for nuclear reactors



United States Patent 3,254,030 PLUTONIUM ENRICHED URANIUM FUEL FOR NUCLEAR REACTORS Gaetan G. J. Michaud, Toronto, Ontario, and Robert G. Hart, Deep River, Ontario, Canada, assignors to Atomic Energy of Canada Limited, Ottawa, Canada, a company incorporated pursuant to the Atomic Energy Control October 1946 No Drawing. Filed July 27, 1962, Ser. No. 213,035

9 Claims. (Cl. 252301.1)

This invention relates to fuels for nuclear reactors and more particularly to nuclear reactor fuels of a mixture of PuO and U0 The following are characteristics of the ideal enriched fuel element for a nuclear reactor:

('1) It should have a low parasitic absorption of neutrons and small changes in reactivity during operation.

(2) The heat output from each element should remain essentially constant during operation to ease the engineering problems of heat removal.

(3) There should be a high utilization of bufriup of the original fissile material.

(4) The fuel elements should be relatively inexpensive to fabricate, and have a high gas retention and corrosionerosion resistance.

(5) The fuel elements should be readily amenable to chemical reprocessing.

(6) The new fissile material formed in the reactor should be separable from the old.

The enrichment of U fuel with Pu is desirable because Pu has no parasitic absorption of neutrons and the heat output of such enriched fuel would remain essentially constant due to the formation of replacement fissile material by neutron capture in the fertile material. Further, Pu being an irradiation by-product is inherently less expensive than natural fissile materials. Thus, the enrichment of U fuel with Pu involves the problem of providing a fuel having a high burning quality which is relatively inexpensive and from which the newly formed irradiation "Pu is separable from the old enrichment, irradiated Pu. It will be understood that after irradiation the old enrichment Pu contains a significant precentage of parasitic Pu 242 and is not a useful fuel, while the newly formed Pu is a useful fuel. If the two are not separated then the old poisons the new and the mixture is not as valuable a fissile material.

The object of the present invention is to provide a useful Pu enriched U fuel for nuclear reactors.

In the irradiation of U the Pu formed by neutron capture in U goes into solid solution with the remaining U, both as oxides, and the solid solution is soluble in nitric acid. on the other hand the solubility of PuO alone in nitric acid is less than about 0.7%. The mechanism of the solid solution formation probably involves diffusion of the similar Pu and U atoms across the contact surfaces of Pu and U oxide particles. The tendency to form solid solution is therefore believed to be related to surface area of the particles, and also temperature, since in general the rate of diffusion will tend to increase with temperature.

It has been discovered that in admixture with U0 PuO in refractory form having a surface area within a specific range will substantially resist the tendency to go into solid solution with U under irradiation conditions.

'To the extent that the Pu does not go into solid solution,

it can be separated from the new irradiation Pu by leaching out the latter with nitric acid. The surface areaof PuO being difiicult to determine experimentally is expressed herein in terms of particle size.

It has been further determined that if more than about 10% of the old Pu remains in the new Pu after leaching 3,254,030 Patented May 31, 1966 that the new Pu contains too much Pu for economic recycling as fuel enrichment.

Broadly this invention relates to a nuclear fuel comprising a mixture of at least by weight U0 and particles of refractory PuO at least about wt. percent having a size between about 44 and about 84011., said PuO being not more than 10% by weight soluble from the fuel mixture in cone. H NO at 60 C. after three hours. i

The tendency to form solid solution is probably also affected by the size and nature of the U0 particles, but other required fuel characteristics limit the desirability of decreasing the surface area and limit the nature of the U0 For example, the fuel must have a high and uniform density and uniformity. Thus, well known forms of U0 particles for example as described in A.E.C.L. Report C.R.C.E. 716-Parts I, II and III are suitable. The desirable density of oxide fuels is at least and more preferably of theoretical density and thus, this requirement and uniformity requirements control the desirability of increasing the particle size of enrichment P1102.

Refractory PuO as defined herein is prepared by pyrolizing PuO at high temperatures. The pyrolysis causes densification and some agglomeration of the oxide powder into frits. The strength and size of the frits is dependent on the oxide source and pyrolysis temperature. The appearance and properties of Pu oxides vary with the source. For example, suitable frit size and strength has been obtained 'at pyrolizing temperatures as low as 850 C. for hydroxide source PuO and about 900 C. for nitrate and oxalate PuO Preferably pyrolizing temperatures are at least 1200 C. for hydroxide P 1300 C. for nitrate Pu0 and 1400 C. for oxalate PuO In general solid solubility with U0 has been found to decrease with increased pyrolizing temperatures, but an optimum pyrolizing temperature appears to have been found for hydroxide P-uO at about 1250 C. and for nitrate PuO at about 1450 C.

It has been observed that hydroxide PuO pyrolized at 550 C. contains very few fines and at 1250 C. the frits are larger. At 1450 C. however, there are more fines than at 1250 C. Nitrate PuO exhibits many fines at 300 C., but very few at 900 C. Above '1550 C. there may be some increase in the number of fine particles. At these higher temperatures the hydroxide and nitrate PuO frits appear to be more brittle and may tend to break-up during compaction in preparing the fuel.

Oxalate PuO exhibits many sub-micron particles at 650 C. pyrolysis, but the number is decreased substantially at 1450 C.

While sulphate PuO pyr-olizes with few fines with frit particle size increasing with pyrolizing temperature, yet it has been found that a relatively high solid solubility is obtained as compared with hydroxide, nitrate and oxalate source P110 It is possible that low frit strength or low density or both are responsible.

The preferred refractory PuO is therefore hydroxide PuO pyrolized at, at least about 850 C. and more preferably 1250 C., nitrate Pu0 pyrolized at, at least about 1300 C. and more preferably at 1450 C. and oxalate PuO pyrolized at, at least about 1400 C.

The maximum desirable enrichment of U0 fuel with Pu0 is about 25 weight percent of PuO and is preferably in the order of about 2 weight percent PuO Expermiental results indicate that for the preparation of a suitable fuel, at least about 80 wt. percent of the refractory Pu0 enrichment frits must be of a size from about 44 to 840p or passing through a No. 20 and retained on a No. 325 US. screen and preferably 30H screen. Unsized oxalate, nitrate and hydroxide PuO powders can give satisfactory frits if pyrolized at the higher temperatures. It has been found desirable to pelletize compact and sinter the P110 powder, (particularly the oxalate P-uO to give a dense frit and then grind to a refractory particle of suitable size before mixing with U The fines can be recycled in this process.

The fuel is prepared by uniformly mixing the U0 and refractory PuO particles, compacting to the desired shape and sintering to a unitary body of high density.

A preferred procedure is illustrated in the following example:

Uranium diode powder was prepared from ammonium diuranate according to the procedure defined in A.E.C.L. report C.R.C.E. 716 parts I, II and III, to give an O/U ratio of 2.260.

Plutonium oxide starting material was prepared according to the procedure described in report AERE-R- 2939 (1959), using ignition temperatures of 650 C., 300 C. and 550 C. for the oxalate, nitrate and hydroxide respectively. The nitrate and hydroxide PuO powders (unsized) were pyrolized at 1450 C. for two hours and then the frits sized to -30+150 U.S. screen. About 50 wt. percent of the nitrate and about 90 wt. percent of the hydroxide PuO particles were of this size. The oxalate PuO was first pressed at 60,000 p.s.i. into pellets and then pyrolized in air at 1450 C. for two hours to give 92% theor. density. This pre-densified PuO was crushed and sized to -30|150 U.S. screen. About 80 wt. percent was in this size range.

The resulting refractory P110 was mixed with the U0 in a mechanical vibrator for 2 minutes to give a Pu0 concentration of 2%. The mixed powder was pressed at 60,000 p.s.i. to give gm. green pellets of 55% theor. density. The green pellets were sintered at 1450 C. for two hours in an inert atmosphere and for 10 minutes in hydrogen and then cooled in hydrogen to give an O/U ratio in the U0 of 2.005 and a density of about 10.6 gm./cm. (about 96.5% theor. density). The heating and cooling rate was 550 C./hour.

The solid solubility was determined by leaching the sintered pellets in conc. nitric acid at 60 C. for 3 hours. The Pu content of the solutions was determined by alpha particle counting. The results are shown in Table I.

TABLE I PuO origin: Wt. percent total Pu dissolved Suitable reducing gases other than hydrogen can be used. The pellets can be pressed at pressures down to about 40,000 p.s.i. The pre-densified sized oxalate P110 is preferred as it gives a very low Pu solubility. The pyrolysis of the PuO may be sufficiently effected in less than two hours. Normally there is no advantage in prolonging the heat treatment beyond about two hours.

We claim:

1. A fuel for nuclear reactors comprising a mixture containing at least 75% by weight of U0 and the remainder refractory P'uO at least 80% by weight of said PuO being of a size from 44 to 840 microns, said PuO being not more than 10% soluble from said mixture in concentrated nitric acid in 3 hours at 60 C., said mixture having a density of at least 90% of theoretical density.

2. A fuel as defined in claim 1 having a density of at least 95% of theoretical density.

3. A fuel as defined in claim 1 containing by weight 98% of U0 and 2% of said PuO 4. A method of preparing a fuel for nuclear reactors comprising the steps of igniting one of the group of Pu compounds consisting of Pu oxalate, Pu nitrate and Pu hydroxide to form PuO pyrolizing said PuO at a temperature of at least 850 C. for the hydroxide PuO at least 900 C. for the nitrate PuO and at least 900 C. for the oxalate PuO to form PuO particles, at least by weight of said PuO particles having a size from 44 to 840 microns, intimately mixing together at least 75 by Weight of mixture of U0 with said PuO particles, compacting and sintering said mixture to a density of at least of theoretical density. I

5. The method as defined in claim 4 in which said PuO ignited from Pu oxalate is compressed at a pressure of at least 40,000 p.s.i. into pellets and pyrolized at a temperature of at least 1400 C.

6. The method as defined in claim 4 in which the PuO is pyrolized at a temperature of at least 1200 C. for the hydroxide PuO and at least 1300 C. for the nitrate P1102.

7. A method of preparing a fuel for nuclear reactors consisting essentially of the steps of igniting Pu oxalate at a temperature of 650 C. to form PuO compressing said PuO into pellets at a pressure of 60,000 p.s.i., pyrolizing said pellets in air at a temperature of 1450 C. for t-wo hours to form refractory PuO crushing said refractory PuO into particles and sizing said particles, intimately mixing with U0 particles, 2% by weight of the mixture, of said P particles having a size of 30,+ U.S. screen, compacting and sintering said mixture to a density of at least 96% of theoretical density.

8. A method of preparing a fuel for nuclear reactors consisting essentially of the steps of igniting Pu nitrate at 300 C. to form PuO pyrolizing said PuO at 1450 C. for 2 hours to form refractory P-uO sizing said refractory PuO to 30+150 U.S. screen, intiamtely mixing 2% by weight of the mixture of said sized refractory PuO with U0 and compacting and sintering the mixture to a density of at least 96% of theoretical density.

9. A method of preparing a fuel for nuclear reactors consisting essentially of the steps of igniting Pu hydroxide at a temperature of 550 C. to form PuO pyrolizing said PuO at 1250 C. for 2 hours to form refractory PuO sizing said refractory P110 to 30',+'150 U.S. screen, intimately mixing 2% by weight of the mixture, of said sized refractory PuO with U0 and compacting and sintering the mixture to a density of at least 96% of theoretical density.

References Cited by the Examiner UNITED STATES PATENTS 2,868,707 1/1959 Alter et a1.

OTHER REFERENCES AEC Report I-IW-69832, June 1961, p. 3.

ABC Report TID-7456 (Book 2), November 1957, pp. 542 and 547.

ABC Document GEAP-3487, Aug. 15, 1960.

Journal of British Nuclear Energy Society, vol. 1, No. 1, January 1962, p. 5.

Nuclear Science Abstracts, vol. 15, November-December 1961, Abstract No. 31813.

Nuclear Science Abstracts,vol. 15, September-October 1961, Abstract No. 25168.

Second Geneva Conference on Atomic Energy, September 1958, vol. 6, pp. 215 and 216.

LEON D. ROSDOL, Primary Examiner.

REUBEN EPSTEIN, CARL D. QUARFORTH,

Examiners.

R. W. MACDONALD, A. G. BOWEN, B. R. PADGETT,

Assistant Examiners. 

1. A FUEL FOR NUCLEAR REACTORS COMPRISING A MIXTURE CONTAINING AT LEAST 75% BY WEIGHT OF UO2 AND THE REMAINDER REFRACTORY PUO2, AT LEAST 80% BY WEIGHT OF SAID PUO2 BEING OF A SIZE FROM 44 TO 840 MICRONS, SAID PUO2, BEING NOT MORE THAN 10% SOLUBLE FROM SAID MIXTURE IN CONCENTRATED NITRIC ACID IN 3 HOURS AT 60*C., SAID MIXTURE HAVING A DENSITY OF AT LEAST 90% OF THEORETICAL DENSITY.
 4. A METHOD OF PREPARING A FUEL FOR NUCLEAR REACTORS COMPRISING THE STEPS OF IGNITING ONE OF THE GROUP OF PU COMPOUNDS CONSISTING OF PUOXALATE, PU NITRATE AND PU HYDROXIDE TO FORM PUO2, PYROLIZING SAID PUO2 AT A TEMPERATURE OF AT LEAST 850*C. FOR THE HYDROXIDE PUO2, AT LEAST 900*C. FOR THE NITRATE PUO2, AND AT LEAST 900*C. FOR THE OXALATE PUO2 TO FORM PUO2 PARTICLES, AT LEAST 80% BY WEIGHT OF SAID PUO2 PARTICLES HAVING A SIZE FROM 44 TO 840 MICRONS, INTIMATELY MIXING TOGETHER AT LEAST 75% BY WEIGHT OF MIXTURE OF UO2 WITH SAID PUO2 PARTICLES, COMPACTING AND SINTERING SAID MIXTURE TO A DENSITY OF AT LEAST 90% OF THEORETICAL DENSITY. 